This invention relates generally to reaction motors and more specifically to a throat structure for arcjet thruster and other reaction engines which includes heat pipes for cooling the throat.
The constriction of the throat of an arc jet or rocket engine produces a particularly difficult problem in cooling. The heat of such a device is concentrated at the narrow throat, and the very configuration of such a throat reduces the surface area available for heat dissipation. Moreover, the temperatures at the throat are very high so that specific materials are needed to withstand the temperatures without adverse effects.
Arcjet thruster engines have even more severe requirements. Although the general configuration of the throat is similar to that of more conventional rockets, the arcjet thruster operates on the basis of an electrical arc heating incoming gases and thereby expanding the gases to such a degree that they leave the throat with a force which provides thrust. The materials at the throat must therefore be suitable to maintain the electric arc and not be destroyed by the arc. Unlike old arc lamp projectors or electric welding systems, an arcjet thruster engine can not include consumable electrodes, so the cooling system must be effective enough to maintain temperatures which prevent erosion of the electrodes.
Prior art cooling arrangements have treated rocket throats like any other cooling problem. Most such throats have been constructed with corrugated walls, coils of tubing, or annular chambers immediately adjacent to the throat to form a heat exchanger through which liquids or gas are pumped to cool the throat. Some arrangements use the rocket fuel itself as the coolant. Other systems include U.S. Pat. No. 3,077,073 by Kuhrt which converts a liquid fuel into gas within such an annular heat exchanger, and U.S. Pat. No. 5,178,514 which uses heat pipes oriented annularly within the shroud ring of a gas turbine engine.
However, all of these prior art heat exchangers still include one limitation which has not been effectively overcome. The remaining problem is the limited area around a rocket throat available to transfer heat from the rocket throat to the heat exchanger. Since all prior art heat transfer systems are dependent on the surface area immediately around the rocket throat to remove heat from the throat, the small size of the throat limits the ability to remove heat, and therefore causes increased temperatures at the throat.
This problem is even more severe for an arcjet thruster engine in which a typical throat can be less than 25 mm in diameter and the order of one millimeter long. All the heat within such an engine is therefore being generated in a space approximately the size of a penny, with only the area of the edge of a penny through which to transfer the heat. Regardless of the material or coolant used, the temperature gradient which results from such a small area of heat transfer can be very severe.